1. Field of the Invention
The present invention relates to a method for producing a polyolefin by polymerizing or copolymerizing at least one olefin in the presence of a novel catalyst system. More particularly, the present invention relates to a method for producing a polyolefin having excellent qualities including powder properties at a very high activity as compared with conventional methods.
2. Discussion of Background
It is already known to use a catalyst system comprising a transition metal compound and an organic metal compound in low pressure polymerization of olefins. Also, a catalyst system containing an inorganic or organic magnesium compound and a transition metal compound is known as a highly active catalyst.
For example, Japanese Examined Patent Publication No. 15110/1977 discloses a highly active catalyst system comprising a catalyst component (A) obtained by reacting magnesium metal with a hydroxylated organic compound or an oxygen-containing organic compound of magnesium, an oxygen-containing organic compound of a transition metal and a halogenated aluminum compound, and a catalyst component (B) of an organic metal compound.
However, activities of these catalyst systems are till insufficient, and polymer particles obtained thereby have various disadvantageous powder properties in that average particle sizes are small, that particle size distributions are broad and that the proportions of fine particles contained in polymer particles are large.
Thus, the above-mentioned disadvantages sometimes provide problems such as a catalyst residue remains in a polyolefin in a large amount, thereby coloring the polyolefin and degrading weather resistance of the polyolefin, and that various troubles are caused at the steps of powder transportation and granulation, thus sometimes making long term-continuous production impossible. Also, in slurry polymerization, separation of particles from a polymer slurry and drying of powder are not easy, and in gas phase polymerization, a production step is blocked by powders. Moreover, in preparation of a polymer by multi-stage polymerization, if a particle size distribution of polymer particles is broad, powders are easily classified at a blending stage of additives or at a transportation stage and a harmful influence is brought on qualities since properties vary depending on a particle size.
It is disclosed in Japanese Examined Patent Publication No. 58367/1987 that a particle size of a polymer can be made large by adding a silicon compound to the starting material of the above-mentioned catalyst component (A), but the catalyst activity and the particle size distribution are not improved.
Under these circumstances, we found and described in Japanese Unexamined Patent Publication No. 309505/1992 that a particle size distribution and a particle size of a polyolefin can be improved by using a catalyst obtained by making ethylene and/or an .alpha.-olefin other than ethylene in contact with and absorbed in a solid component containing titanium, but catalyst activity is still poor.
An object of the present invention is to improve catalyst activity without impairing the above-mentioned properties of the catalyst disclosed in Japanese Unexamined Patent Publication No. 309505/1992.
In order to solve the above-mentioned problems, we have studied and discovered that a satisfactory catalyst activity as well as satisfactory powder properties of a polyolefin product can be obtained by using a solid titanium catalyst component obtained by (i) precipitating particles by reacting at least one aluminum halide compound with a homogeneous solution containing magnesium, titanium and alkoxy groups, in such an amount as to provide the halogen of the aluminum halide compound to the alkoxy groups contained in the homogeneous solution within a specific molar ratio range, and (ii) further treating the resultant mixture containing the precipitated particles with at least one aluminum halide compound. The present invention has been made on the basis of this discovery.